A particularly important field of application for the present invention is in connection with refiners for making fiber- or papermaking pulp from wood chips or similar cellulosic material. Refiners of the disk type are formed with a refiner gap extending in the radial direction between the refining elements, which gap starts from a central feed zone for the raw material where the centrifugal force is relatively low. The centrifugal force affecting the material to be refined therein increases very strongly with increasing radius. In order to prolong the dwell time in the outer portion of the refiner gap, the outer portion thereof can be formed so as to extend angularly in relation to the radial direction, so that only part of the centrifugal force is permitted to affect the material to be refined in the flow direction of the refiner gap. While steps have thus been taken in order to limit the flow rate of the material to be refined in the radially or outer portion of the refiner gap in connection with this design the effect of the centrifugal force in the central feed zone is low, so that the feed to the outer angular portion of the refiner gap is not as intense as would be desired in order to achieve the maximum capacity of the refiner. Attempts have been made to introduce mechanical devices to promote the discharge in the central feed zone, which, however, are not effective, because, inter alia, they must be exchanged relatively often due to their rapid wear.
An object of the present invention is to eliminate these problems and thereby bring about effective feed of the material to be refined from the inner radial portion of the refining zone to the outer angular portion.